In accordance with development of portable apparatuses having small size, light weight and high performance, demand has arisen for a lithium secondary battery of high capacity. In connection with this demand, studies have been performed on a negative electrode material having a capacity exceeding 372 mAh/g, which is the theoretical capacity of graphite, which has conventionally been employed as a negative electrode material for producing a lithium secondary battery.
Specifically, a non-carbonaceous material for a negative electrode exhibiting larger capacity such as a material containing silicon, tin, aluminum and tungsten element has been proposed as a replacement for such a graphite negative electrode material.
For example, JP-A-2000-173612 (the term “JP-A” as used herein means Japanese Laid-Open (kokai) Patent Application No.) discloses a negative electrode material prepared by coating silicon elemental particles with a silicon-containing solid solution layer or an intermetallic compound layer, and depositing a carbonaceous material containing fibrous carbon onto a portion or the entirety of the surfaces of the thus-coated particles. JP-A-2000-357515 (U.S. Pat. No. 6,300,013) discloses a mixture of a silicon compound with a carbon material as a negative electrode material.
However, when lithium ions are intercalated into or released from such a non-carbonaceous negative electrode material; i.e., when doping or dedoping of lithium ions is performed, particles of an active substance undergo a great change in volume, and microspaces are generated between the active substance particles, leading to a quantitative reduction in a portion of the electrode material that is effectively employed for storing electrical charges. Specifically, cracking occurs in the electrode material as a result of such volume change, the active substance particles are micronized, and microspaces are generated between the thus-micronized particles, leading to interruption of electronic conduction networks caused by contact between the particles, and a quantitative increase in a portion of the electrode material that cannot participate in electrochemical reaction. Conceivably, this phenomenon causes problems, including decrease in charging/discharging capacity and an increase in internal electrical resistance.
As described above, when lithium ions are intercalated into or released from the non-carbonaceous negative electrode material, the volume of the active substance particles changes greatly. Therefore, the electrode material has problems that the material is considerably impaired by repeating charging/discharging cycles, and that its internal electrical resistance (particularly at low temperature) is increased.
The electrode material disclosed in JP-A-2000-173612 is prepared by depositing a carbonaceous material containing fibrous carbon onto silicon particles serving as nuclei. Therefore, when the electrode material undergoes charging/discharging cycles, the particles constituting the material are micronized, and thus each particle fails to maintain its shape, and the particles are separated from one another, leading to problems in terms of, for example, cycle characteristics and irreversible capacity.
In the electrode material disclosed in JP-A-2000-357515 (U.S. Pat. No. 6,300,013), in which the ratio between the average particle size of silicon compound particles and that of carbon material particles is regulated, doping/dedoping between lithium ions and the silicon compound particles is performed within microspaces formed by the carbon material particles, which have a particle size greater than that of the silicon compound particles. When this electrode material undergoes repeated charging/discharging cycles, these particles are separated from one another, leading to problems in terms of, for example, cycle characteristics and irreversible capacity.
On the other hand, JP-A-2002-8652 discloses a negative electrode material prepared by depositing finely divided silicon particles onto graphite powder and forming a carbon coating on the graphite powder. JP-A-2002-255529 discloses a carbonaceous material prepared by depositing, onto graphite particles, composite particles formed of silicon and an electrically conductive carbon material, and coating the composite particles with amorphous carbon.
However, the electrode material disclosed in JP-A-2002-8652 has problems that expansion or shrinkage of the silicon particles occurs along with charging/discharging cycles, and that contact between the silicon particles and the graphite powder fails to be maintained, and thus electrical conductivity is lowered, leading to deterioration of cycle characteristics.
In the carbonaceous material disclosed in JP-A-2002-255529, silicon and an electrically conductive carbon material (e.g., carbon black) are completely coated with amorphous carbon. Therefore, the carbonaceous material involves the following problems that electrical conductivity is low and large-current characteristics are impaired, and that contact between the particles fails to be maintained due to the expansion or shrinkage of the particles in charging/discharging, leading to deterioration of cycle characteristics.